In today’s medical science studies, many tumor markers are being used to monitor cancer cell proliferation, but the number of assays for analysis of these markers are few. The aim of this study was to find an easier and more time-efficient way to measure the activity of a specific tumor marker called tymidine kinase 1 (TK1). This tumor marker is an important enzyme involved in cell proliferation and is a key enzyme in the salvage pathway. TK1 activity is related to the occurrence of hematological malignancies and cell activity and therefore have been used as a marker when monitoring this group of patients in treatment. Measurement of the enzyme activity in this study was performed by using an oligonucleotide assay. Detection of the enzyme activity in whole blood and in plasma has not previously been shown. The TK1 activity measured in whole blood and plasma correlated with TK1 activity measured in serum (R2=0,8651 and R2 =0,9845, respectively). It was found that it is possible to determine the TK1 activity in whole blood but only if the activity was measured on the same day as the blood samples were taken. The results shows that the activity measurement of TK1 in plasma and whole blood can be used as a marker to verify patients' therapy in cancer care. This study is only the beginning and further investigations should be made in the future to determine if the method that is subject to this study has the requested effects.